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"For a plane or bird to fly, its wings must produce enough lift to equal its weight. "

I got that excerpt from "https://www.sciencelearn.org.nz/resources/300-wings-and-lift".

My questions: (1) Can an aircraft wing create lift that is greater than the weight of the aircraft? (2) If the answer to question (1) is yes, then what happens in such a situation?

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    $\begingroup$ The aircraft climbs. This implies that to fly higher than ground level a flying thing MUST generate lift in excess of its weight at least some of the time $\endgroup$ – slebetman Jan 11 at 5:48
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    $\begingroup$ It's not clear from your question that you understand the principles of flight. If lift was not greater than weight then how do you think an aircraft could rise in the air? $\endgroup$ – Transistor Jan 11 at 14:32
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    $\begingroup$ @Transistor: By having a thrust-to-weight ratio greater than 1 and pointing the engines downward, for example. (Yes, this is not aerodynamic lift, but you started your comment with the assumption that the OP doesn't understand lift, and then proceed to make an argument based on lift, so be prepared for that to … ahem … fly over their head.) $\endgroup$ – Jörg W Mittag Jan 11 at 17:23
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    $\begingroup$ @Jörg W Mittag: Sure, but very few airplanes (or even helicopters) have their engines pointed downwards. Of course neither propellor planes nor helicopters would get very far if their propellors/rotor blades (which are just specialized wings) didn't produce "lift". And jet engines tend to have lots of little "wings" inside them... $\endgroup$ – jamesqf Jan 11 at 17:42
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    $\begingroup$ @slebetman -- no, see -- aviation.stackexchange.com/questions/40921/… -- "Clearly, Lift is less than Weight in a powered climb." $\endgroup$ – quiet flyer Jan 13 at 14:15
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Yes, a wing can (given sufficient forward speed and angle of attack) generate lift greater than the weight of the aircraft. As with any "unbalanced" force, this will result in an acceleration of the airplane in the direction of the lift, according to Newton's Second law. $$\mathbf F=m~\mathbf a$$

Please note, the entities in bold face are vector quantities.

This is what happens during any positive $\mathrm G$ maneuver, like a pullout from a dive, a loop, etc.

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    $\begingroup$ Or even a simple take-off or climb. $\endgroup$ – Ray Butterworth Jan 11 at 1:20
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    $\begingroup$ @Ray: Strictly speaking the lift exceeds weight only while transitioning to a higher rate of climb; when a steady climb is achieved the lift matches the weight again. $\endgroup$ – A. I. Breveleri Jan 11 at 2:10
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    $\begingroup$ @A.I.Breveleri In fact, because the engines are pointed somewhat downwards, the lift will actually be less than the weight. $\endgroup$ – SomeoneSomewhereSupportsMonica Jan 11 at 9:03
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    $\begingroup$ To be even more precise, when the rate of climb is constant, be it positive or negative, some drag will be present on the up/down axis. When the climb is positive lift will need to be a bit higher than weight, to win this additional force. $\endgroup$ – Vladimir Cravero Jan 11 at 17:16
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    $\begingroup$ @VladimirCravero That is true, and besides that the lift is also not aligned with gravity anymore. Lift is defined as being perpendicular to drag, not necessarily aligned with gravity. Altough it is usually the case, lift does not always have to be larger than gravity when climbing, some jet fighters are so powerful they can hang vertically in the air with almost no velocity and thus with nearly no lift. $\endgroup$ – Orbit Jan 11 at 19:31
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Yes, otherwise airplanes would be unable to go upwards into the sky.

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    $\begingroup$ Perfect case of reductio ad absurdum. I would rather use gliders to make it even more undisputable (the wings can lift the airframe even when no engine is helping). $\endgroup$ – kuroi neko Jan 12 at 1:05
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    $\begingroup$ I like the simplicity of this answer far more than the other answers. They make it sound far more complex than it is. $\endgroup$ – Notts90 supports Monica Jan 12 at 9:05
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    $\begingroup$ This answer is simple, but it's a bit oversimplified. It's entirely possible to climb without having lift > weight. Rockets, for example, use thrust instead of aerodynamic lift to counter the gravitational force. The simplest way to put it that is still completely correct would be to say, "In order for the aircraft to accelerate upwards, it must be able to, at least briefly, generate an upward force greater than its weight." Once the climb has been established, the net upward force can (and, in the case of an airplane, typically will) return to being equal to the weight. $\endgroup$ – reirab Jan 13 at 8:49
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    $\begingroup$ @reirab Hence kuroi nekos suggestion. $\endgroup$ – Taemyr Jan 13 at 10:29
  • $\begingroup$ Gliders generate lift, but except when coming out of a dive (with lots of speed), they only gain altitude when there's an updraft to take advantage of. Sans motor/propeller, they can't generate airspeed (other than by diving) $\endgroup$ – Carl Witthoft Jan 13 at 13:08
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Furthermore to @Zeiss' answer, whenever an aircraft is steady-state banked, the lift will be greater than its weight. However, its speed will be constant; instead, the acceleration is centripetal and results in a circular turn.

Edit, clarification on pull up maneuver:

When an aircraft is pitched up via pitch control, and after the short-period mode settles (a few seconds at most), it will gain a lift imbalance greater than its weight due to higher angle of attack (AOA). Similar to a banked turn, since the net force is perpendicular to the horizontal velocity, it will be centripetal and results in a vertical circular motion. This is the early stage of the maneuver.

Since a higher AOA has higher drag, the airspeed will decay toward a new and lower trim speed. At the same time, as airspeed decreases, the net lift also decreases, so there is less force imbalance. As the decreasing airspeed undershoots the new trim speed, the aircraft will pitch down again. This cyclical exchange of energy is called phugoid and usually persists for a while (dozens of minutes to an hour) if left unchecked.

Eventually, however, the phugoid dies down (phugoid is usually stable in non-transonic regime) and the airplane is flying at the new trim speed, with lift equal to weight once again. If the airplane is in the front of the power curve, it will climb at steady-state; otherwise, it will descend.

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A kite is a simple aircraft, generating lift. The vertical component of the pull you feel on the string is any resultant lift greater than the weight of the aircraft, the horizontal component being the drag. In a non-tethered aircraft, excess lift causes the aircraft to "rise", or more precisely, causes the flight path to curve upwards.

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  • $\begingroup$ See -- aviation.stackexchange.com/questions/40921/… -- "Clearly, Lift is less than Weight in a powered climb." $\endgroup$ – quiet flyer Jan 13 at 14:17
  • $\begingroup$ I've edited as per my suggestion. You are welcome to roll back if you feel the edit does not respect the intent of your answer. $\endgroup$ – quiet flyer Jan 13 at 20:17
  • $\begingroup$ @quietflyer the intent was to supplement the other answers' more theoretical descriptions with a physical sensation the OP may well know, as many people learn better that way, with all these comments it rather spoils the effect. Oh well. $\endgroup$ – Pete Kirkham Jan 13 at 21:13
  • $\begingroup$ Ok comments removed. If you want to roll it back all the way I'll leave it alone. $\endgroup$ – quiet flyer Jan 13 at 21:45
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The lift a wing CAN generate is a design feature of the wing.

The lift a wing does generate is a feature of the weight and the flight path of the aircraft.

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    $\begingroup$ The first part is correct, but the second part is not. Lift produced is a function of speed and angle of attack. Weight does not contribute to generating lift, it is the opposing force that must be overcome by lift to fly. $\endgroup$ – Michael Hall Jan 12 at 16:14
  • $\begingroup$ In addition to what Michael said, flight path doesn't (directly) influence wing lift generation, either. Angle of attack (angle between air flow and wing chord line) does, though. $\endgroup$ – reirab Jan 13 at 8:45
  • $\begingroup$ More than one way to skin a cat; I think the answer is fine. If you know the weight and the flight path, then you know the lift. Consider that the pilot (or autopilot) is also in the loop, manipulating the controls, and thus indirectly the angle-of-attack, to obtain the desired flight path. Therefore it's arguably a true statement to say that "the lift a wing does generate is a feature of the weight and the flight path of the aircraft." Is the statement claiming that flight path is CAUSING the lift rather than vice versa? I'd say no. $\endgroup$ – quiet flyer Jan 13 at 22:37
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If you have an aircraft with 20 tons of cargo, and the total weight of the aircraft, including cargo, is 60 tons, then clearly the wings are capable of producing 60 tons of lift. If you fly the same plane without cargo, then that's not going to affect the amount of lift the wings can produce. So clearly there are situations where the wings can produce more lift than the weight of the aircraft.

Something else that shows that aircraft must be sometimes operating below their maximum lift is that when an airplane takes off or lands, it's not moving at its maximum speed. Since lift increases with speed, that means that its lift is significantly crippled. If an airplane has enough lift to fly at runway speeds, then it has more than enough lift to fly at cruising speed.

In addition, an aircraft is going to need a margin to climb, turn (one of the reasons airplanes bank when turning is that the lifting force of the wings can then be applied to centripetal force, aka turning the aircraft), etc. Tell any pilot that you want them to fly a plane with a maximum lift equal to, or only slightly above, the weight of the plane, and unless they have a death wish they'll be running the other way.

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  • $\begingroup$ Some parts of your answer appear to be using "lift" to mean something more like "potential lift", meaning the amount of lift that would be generated at any given airspeed if we instantly snapped the wing to the max-lift angle-of-attack. Does lift-- real lift, not "potential" lift-- really increase with airspeed? If yes, then what are you holding constant, and what is the flight path that results? $\endgroup$ – quiet flyer Jan 13 at 22:30

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